When Tsar Peter the Great founded the former Russian capital city of Saint Petersburg more than 300 years ago, he intended it to provide a "window on the Baltic." Turns out, that window was more of a screen door. To protect itself, the city built a dam that took almost 300 years and $3.85 billion to complete.

You see, Saint Petersburg is constructed on drained marshland, isles, and lowlands along the Neva River, situating no less that 33 percent of this 4.6 million-citizen city smack dab in the middle of flood country. In fact, since 1703, the Neva river has backed up and flooded the city almost every autumn—some 308 separate events—and sometimes twice annually. The worst of these floods occurred in 1824 when a 12-foot storm surge inundated low-lying neighborhoods, destroying 450 homes and killing hundreds. And it's not just lives and private property that are at stake; many of Saint Petersburg's (nay, Russia's) most prized national cultural icons and monuments are located on flood-prone streets.

The flooding became an issue almost immediately after the city was built, and in 1727, Count Zakhar Chernyshov put forth an initial plan to tame the Neva using a series of dams and locks. Chernyshov didn't live to see his design built, but in 1979—nearly 252 years later—the Soviets finally resurrected the plan. The resulting Saint Petersburg Flood Prevention Facility Complex would become the longest running public works project in USSR history.

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Even when the iron curtain fell in the early 1990s, the dam was only about 65 percent complete—again, thanks to the Soviets. With the demise of the Soviet Union, there wasn't much capital floating around so the entire project was once again shelved indefinitely. It wasn't until Prime Minister Vladimir Putin came to power and personally ordered its completion in 2005 that construction resumed. The complex was finally finished in 2011, some 32 years and 109 billion rubles ($3.85 billion) later.

Informally known as the Saint Petersburg Dam, the Saint Petersburg Flood Prevention Facility Complex (or SPFPFC—aka the worst acronym ever) spans nearly 16 miles, separating the Neva from the rest of the Gulf of Finland. A series of eleven dams and locks run northward from the coast at Lomonosov up to Kotlin Island before bearing East towards Sestroretsk.

The steel and concrete structure (2,600,000 cubic yards of concrete and 110,000 tons of steel, specifically) stands 26 feet above the waves and can withstand storm surges up to 16 feet in height. Each dam is nearly 100 feet wide—enough to fit a six lane highway on top as well as a 3/4 mile underwater tunnel at the South lock, the longest such tunnel in Russia. The six locks consist of 78-foot wide sluices designed to swing open and flush the outflow of the river—but close when flooding is imminent to prevent the river's back-flow from entering the city. The structure also contains two locks for boat passage and 30 purification centers for cleaning up the outgoing fluids before they hit the gulf.

This massive structure has already begun paying for itself. In November of 2011, less than four months after it was commissioned, the dam held storm surges at just 3.9 feet above sea level, well below the 4.8-foot mark that denotes an official flood. That's right—it halted the city's 309th flood outright and prevented an estimated 1.3 billion rubles ($38 million) in damages. And given that a twelve foot surge causes, on average, 176 billion rubles ($5.29 billion) in damage, the cost of the dam is between 50 and 70 times less expensive the the Russian people over the course of its service than cleaning up after doing nothing. [Wikipedia - St Petersburg Dam (Ru) - Water Technology - Reuters - Image: AP Images - Highway: velopiter.spb.ru]